Peter J. Moran

Roper Resonance in 2+1 Flavor QCD

Abstract The low-lying even-parity states of the nucleon are explored in lattice QCD using the PACS-CS collaboration 2 + 1 -flavor dynamical-QCD gauge-field configurations made available through the International Lattice Datagrid (ILDG). The established correlation-matrix approach is used, in which various fermion source and sink smearings are utilized to provide an effective basis of interpolating fields to span the space of low-lying energy eigenstates. Of particular interest is the nature of the first excited state of the nucleon, the N 1 2 + Roper resonance of P 11 pion–nucleon scattering. The Roper state of the present analysis approaches the physical mass, displaying significant chiral curvature at the lightest quark mass. These full QCD results, providing the worldʼs first insight into the nucleon mass spectrum in the light-quark regime, are significantly different from those of quenched QCD and provide interesting insights into the dynamics of QCD.

Over-improved stout-link smearing

A new over-improved stout-link smearing algorithm, designed to stabilize instanton-like objects, is presented. A method for quantifying the selection of the over-improvement parameter,

The Realistic Lattice Determination of alpha(s)(M(Z)) Revisited

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String effects and the distribution of the glue in static mesons at finite temperature

, is demonstrated. The new smearing algorithm is compared with the original stout-link smearing, and Symanzik improved smearing through calculations of the topological charge and visualizations of the topological charge density. We find the incorporation of improvement in stout-link smearing to be essential for the accurate study of QCD vacuum structure.

Low-lying odd-parity states of the nucleon in lattice QCD

We revisit the earlier determination of {alpha}{sub s}(M{sub Z}) via perturbative analyses of short-distance-sensitive lattice observables, incorporating new lattice data and performing a modified version of the original analysis. We focus on two high-intrinsic-scale observables, log(W{sub 11}) and log(W{sub 12}), and one lower-intrinsic-scale observable, log(W{sub 12}/u{sub 0}{sup 6}), finding improved consistency among the values extracted using the different observables and a final result, {alpha}{sub s}(M{sub Z})=0.1192{+-}0.0011, {approx}2{sigma} higher than the earlier result, in excellent agreement with recent nonlattice determinations and, in addition, in good agreement with the results of a similar, but not identical, reanalysis by the HPQCD Collaboration. A discussion of the relation between the two reanalyses is given, focusing on the complementary aspects of the two approaches.

Impact of dynamical fermions on QCD vacuum structure

The distribution of the gluon action density in mesonic systems is investigated at finite temperature. The simulations are performed in quenched QCD for two temperatures below the deconfinement phase. Unlike the gluonic profiles displayed at T = 0, the action-density isosurfaces display a prolate-spheroid-like shape. The curved width profile of the flux tube is found to be consistent with the prediction of the free bosonic string model at large distances.

Structure and flow of the nucleon eigenstates in lattice QCD

M. Selim Mahbub, Waseem Kamleh, Derek B. Leinweber, Peter J. Moran, and Anthony G. Williams

SU(3) center vortices underpin confinement and dynamical chiral symmetry breaking

We examine how dynamical fermions affect both the UV and infrared structure of the QCD vacuum. We consider large 28{sup 3}x96 lattices from the MILC collaboration, using a gluonic definition of the topological charge density, founded on a new over-improved stout-link smearing algorithm. The algorithm reproduces established results from the overlap formalism and preserves nontrivial topological objects, including instantons. At short distances we focus on the topological charge correlator, , where negative values at small x reveal a sign-alternating layered structure to the topological-charge density of the QCD vacuum. We find that the magnitudes of the negative dip in the correlator and the positive contact term are both increased with the introduction of dynamical fermion degrees of freedom. At large distances we examine the extent to which instanton-like objects are found on the lattice, and how their distributions vary between quenched and dynamical gauge fields. We show that dynamical gauge fields contain more instanton-like objects with an average size greater than in the quenched vacuum. Finally, we directly visualize the topological charge density in order to investigate the effects of dynamical sea-quark degrees of freedom on topology.

Wilson mass dependence of the overlap topological charge density

M. Selim Mahbub, Waseem Kamleh, Derek B. Leinweber, Peter J. Moran, and Anthony G. Williams

Stout-link smearing in lattice fermion actions

The mass function of the nonperturbative quark propagator in SU(3) gauge theory shows only a weak dependence on the vortex content of the gauge configurations. Of particular note is the survival of dynamical mass generation on vortex-free configurations having a vanishing string tension. This admits the possibility that mass generation associated with dynamical chiral symmetry breaking persists without confinement. In this presentation, we examine the low-lying ground-state hadron spectrum of the pi, rho, N and Delta and discover that while dynamical mass generation persists in the vortex-free theory, it is not connected to dynamical chiral symmetry breaking. In this way, centre vortices in SU(3) gauge theory are intimately linked to both confinement and dynamical chiral symmetry breaking. We conclude that centre vortices are the essential underlying feature of the QCD vacuum.